1. Field of the Invention
The present invention relates to a high frequency device used for MEMS (Micro Electro Mechanical Systems).
2. Description of the Related Art
Along with miniaturization of Si-CMOS (Complementary Metal Oxide Semiconductor), transistor shielding frequency has been drastically improved up to 100 GHz or more. Thereby, it is needless to say that application to several GHz band mainly used in current wireless devices is enabled, and in addition, it is prospective that application to 60 GHz or more millimeter waveband will be enabled. Such an application is a so-called RF-CMOS circuit. By integrating a base band section and an RF front end section into one chip by the CMOS, largely lowered cost is allowed to be realized, and thus active development of the RF-CMOS circuit has been made.
One of tasks in developing the RF-CMOS circuit is, for example, an inductor. In the RF-CMOS, an Si substrate having lower resistance (from several to several kωcm both inclusive) than that of a semi-insulating GaAs substrate used in compound semiconductor is used. Thus, in the case where the inductor is formed on the substrate, capacitive coupling with the substrate, eddy current loss and the like arise. In the result, Q value of the CMOS on-chip inductor is decreased, which is one of obstacles to realizing low power consumption and low cost of the RF-CMOS circuit.
As a trial to improve the Q value of high frequency elements including the inductor, the following trial has been made. That is, by removing part of the substrate, parasitic capacity between the high frequency element and the substrate is tried to be decreased, or loss by an eddy current is tried to be suppressed. For example, the following example has been reported (for example, C. Y. Chi and G. M. Rebeiz, “Planer microwave and millimeter-wave lumped elements and coupled-line filters using micromachining techniques,” IEEE Trans. Microwave Theory Tech., vol. 43, No. 4, pp. 730-738, 1995). In the example, an inductor is provided on a membrane formed by etching from the rear face of a Si substrate by potassium hydrate. Thereby, resonance frequency is improved, and a high Q value (from 50 to 60 both inclusive at from 30 to 40 GHz both inclusive) is realized. Further, there is another example as follows (for example, Japanese Unexamined Patent Application Publication No. 2002-222912). In the example, in a high frequency MCM (Multi Chip Module), a substrate is selectively removed by DRIE (Deep Reactive Ion Etching) from the rear face of the substrate, and a filter and an antenna are formed on an organic thin film (membrane) composed of benzocyclobutene (BCB) or the like. Thereby, acute frequency characteristics with small loss are realized.